Stacks away!

Some major, and sad updates this time around:

MAJOR CHANGES:

• Added Stack data structure, made using Singly-Linked List defined in sl_list.hpp. (woohoo recycle trash baby!)
• Removed a few constructors from node and double_node classes, since they seemed to be way too specific and clunky to use
• Removed tail node pointers from sl_list and dl_list` classes, since I don't know how to code them properly, and they kept causing bugs :((
• Optimized a few functions to make up for lost tail nodes

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MINOR CHANGES:

• Added more comments! (as if I needed any more after all the Doxygen I put in there...)